1. Field of the Invention
The present invention relates to film gates for image projectors and more particularly to such projector film gates having an improved film-cooling structure.
2. Description of the Prior Art
In apparatus which project film images, e.g., slide or motion picture projectors, microfilm readers or projection printers, a problem is usually presented because the high intensity radiation from the projection light source heats the film and projection gate structure. This can cause film buckling, which displaces the film image from the projection plane, and in severe cases can cause damgage to the film or film image.
It is a generally well known technique to cool film at projection gates by directing air toward it. In one approach of this type, high velocity air nozzles, which are spaced out of the optical projection paths extending to and from the projection zone, direct streams of air toward the film. However, as the amount of heat which must be transferred from the projection zone increases, it becomes very difficult to design a nozzle system that will provide adequate cooling without nozzle portions optically interfering in the image projection path. A related problem is that air directing structure around the edges of the projection zone decreases the space available for properly supporting the film in the projection plane.
An alternative cooling approach is to utilize air directing structure that is light-transparent and thus can be located in the optical projection path, close to the film plane. Although such structures transmit light, they absorb other wavelengths of the radiation emitted by the projection source and thereby add to the heat load in the projection zone. Further, such light-transparent structures must be reasonably close to the film plane to achieve reasonable cooling efficiency; and, in such closely-spaced location, the dirt or scratches which accumulate on the light-transparent structures are within the depth of focus of the projection lens and cause degradation of the projected image.